TABLE OF CONTENTS In principle, the design of internal gear teeth is the same as that for spur gears. Any of the basic rack forms used for spur gears may be applied to internal gears. The internal gear drive, however, has several limitations; not only all those which apply to spur gears but also several others which are peculiar to internal gears. As with spur gears, in order to secure effective tooth action, interferences must be avoided. The possible interferences on an internal gear drive are as follows:
First: involute interference. To avoid this, all of the working profile of the internal tooth form must be of involute form.
Second: tip interference. This exists when the tips of the spur pinion interfere with the tips of the internal gear as the teeth come into and go out of mesh. To avoid this, the size of the spur pinion must be a sufficient amount smaller than the size of the internal gear.
Third: fillet interference. This exists when the tips of the teeth of one member interfere with the fillets at the roots of the teeth of the mating member. Proper tooth proportions must be selected to avoid this interference.
Another limitation as regards the assembly of the drive must sometimes be considered. When the difference in the numbers of teeth between the spur pinion and internal gear is small, the tooth action may be correct, but it may not be possible to assemble the pair except by sliding them together in an...
